This invention relates to hardenable unsaturated polyester resin compositions.
Hardenable unsaturated polyester resin compositions are widely used not only as sheet mold compositions (SMC) and bulk mold compositions (BMC) but also for wet molding methods such as the so-called matched die method as well as the RIM method which is a method of injecting resin.
Thermosetting unsaturated polyester resins are known to have large molding shrinkage as they harden at the time of molding. In order to improve the accuracy in measurements as well as other physical characteristics such as impact strength of their molded products, it has been common practice to add thermoplastic high-molecular compounds to such unsaturated polyester compounds. If these compounds are simply mixed together to make a composition, however, the surface quality of its molded products becomes extremely poor and desired levels of improvement in reducing shrinkage and increasing impact strength can hardly be attained. Moreover, such compositions have poor workability in molding processes because they generally do not have sufficient compatibility and/or dispersibility.
According to a widely used conventional method, carboxyl groups are introduced into the molecules of high-molecular compounds to be added to thermosetting unsaturated polyester resins so as to combine these carboxyl groups with the carboxyl end groups of the thermosetting unsaturated polyester resins through an oxide or hydroxide of an alkaline earth metal such as magnesium oxide. Some effects are observed in this case if one uses, as the high-molecular compound to be added, a copolymer of methacrylic acid or acrylic acid and methyl methacrylate, vinyl acetate or styrene. With compositions of this kind, however, one can hardly expect any improvement in physical characteristics of the type mentioned above for their molded products.
In order to improve the impact strength of molded products, one may think of diene polymers as the high-molecular compounds to be added. If such compounds are used in a similar manner, however, lustre spots become conspicuous on the surface of the molded products obtained from such compositions. Such products are inferior also in other physical characteristics, showing no advantageous effects of introducing carboxyl groups into diene polymers. Although diene polymers with carboxyl groups in them are soluble in a styrene monomer which is a monomer component used for hardenable unsaturated polyester resin compositions, they separate into layers extremely quickly if there is in coexistence an .alpha., .beta.-ethylenically unsaturated polyester or the like. This is probably because compatibility and/or dispersibility is so poor between diene-type polymers with carboxyl groups and thermosetting unsaturated polyester resins that, even if magnesium oxide or the like is added to them, a large portion of them becomes separated before their combination through salification, thereby preventing improvement of the physical characteristics of molded products.
In order to improve compatibility and dispersibility with thermosetting unsaturated polyester resins, compositions containing block copolymers with polystyrene block have been considered (Japanese Patent Publications Tokkai 53-74592 and 60-99158). Although they do succeed to some extent in improving compatibility and dispersibility, molded products obtained from these compositions are inferior regarding molding shrinkage and in particular regarding impact strength because use is made of block copolymers depending on polystyrene which is basically not sufficiently tough.